A MOLLUSCAN ASSEMBLAGE FROM THE WACCAMAW FORMATION, NORTHEASTERN SOUTH CAROLINA: DIVERSITY AND STRATIGRAPHIC IMPLICATIONS

This study investigated diversity of a mollusc assemblage from the Waccamaw Formation exposed on the IntraCoastal Waterway near North Myrtle Beach , Horry Co., South Carolina. Three beds at the locality were collected as part of a Research Experiences for Undergraduates study; it was unclear whether the beds should be assigned to the upper or lower Waccamaw. Students in an Invertebrate Paleontology class at University of North Carolina Wilmington processed and analyzed three samples from the lowermost bed, a blue-gray coarsening-upward sand rich in molluscs and corals. Samples were sieved through a quarter-inch mesh and bivalves with beaks and gastropods with apices were picked, sorted and identified to genus level. Diversity was analyzed using Holland’s Analytic Rarefaction (http://www.uga.edu/~strata/software/) and PAST (http://folk.uio.no/ohammer/past/).

The combined samples included ~1800 bivalve and ~100 gastropod specimens. The smallest sample (~500 bivalves) included 29 genera; the other samples each contained 32 genera. Samples did not differ in richness when rarefied to the same sample size. Richness for the three samples combined was 38 bivalve genera. All samples showed a low dominance for the bivalve assemblage (0.07 – 0.08); equitability was ~ 0.83. Gastropods were far less abundant; richness ranged among samples from 7 – 11 genera, with 19 genera in the combined samples. Dominance was greater for gastropods (0.24); Crepidula and Prunum were the dominant gastropods.

Richness of the bivalve assemblage was compared to that of Plio-Pleistocene localities in North Carolina; richness declined through the Plio-Pleistocene in a series of extinction pulses. (Gastropod richness was not compared due to small sample size.) Based on rarefaction to a sample size of 1600 (size of smallest sample compared), rarefied richness (37.3 genera) was significantly less than in the Duplin Formation (Lumber River near Lumberton; 56 genera) and the lower Waccamaw near Old Dock (48 genera) and significantly greater than that of the upper James City Formation on the Neuse River near James City (27 genera). Rarefied richness most closely matches that of the (lower?) James City Formation at Lee Creek Mine, Aurora, NC (37.5 genera). Based on richness, the South Carolina assemblage may represent the lower Waccamaw Formation.